1. Field of the Invention
This invention relates generally to phased array antennas and more particularly to an antenna configuration with tiled overlapping subarrays.
2. Description of Related Art
The use of overlapped subarrays (OLSAs) to generate multiple simultaneous receive beams and to effect time delay networks for wide instantaneous bandwidth applications is well known in the art. A representation of this architecture is shown in FIG. 12. The preferred filter function to simultaneously steer the antenna beam and avoid grating lobes is a gate function. The farfield radiation pattern of a truncated sinc weighting function applied to a subarray approximates the gate function in the far field. The implementation of OLSAs in an antenna dictates a tiled architecture. An article by R. Mailloux, xe2x80x9cAn Overlapped Subarray for Limited Scan Applicationxe2x80x9d, IEEE Transactions on Antennas and Propagation, May 1974, which is hereby incorporated by reference in its entirety, describes a technique for producing an OLSA pattern for one plane.
Traditionally, for a 2 dimensional (2D), 2:1 overlapped subarray antenna, the beams from four adjacent tiles are combined with analog and/or digital devices to form one 2D OLSA. An article by E. DuFort, xe2x80x9cConstrained Feeds for Limited Scan Arraysxe2x80x9d, IEEE Transactions on Antennas and Propagation, Vol. AP-26, No. 3, May 1978, describes beamforming techniques for OLSAs, which is hereby incorporated by reference in its entirety.
Analog beamforming requires complicated and lossy manifolds with N2 ports per antenna, where N is the number of OLSAs in each dimension. For example, four tiles feed data into a single receiver, and each tile feeds four receivers. The interdependence of the individual components in the analog OLSA architecture results in difficulties with channel equalization. Inaccessibility of the orthogonal beams that form the four parts of an OLSA hinders calibration and tightens manufacturing tolerances. FIGS. 13 and 14 show the complex physical interconnects necessary for analog beamforming in conventional systems.
Referring to FIG. 13, a typical 2D 2:1 OLSA is shown. The tile ports and receivers locations are superimposed over the OLSA architecture. Area 210 shows a portion of the antenna that includes four receivers and portions of nine tiles. FIG. 14 is a schematic view of area 210 that shows the physical interconnections among the tiles 231-239. Each tile 231-239 has four ports that correspond to a quadrant of the tile as shown in FIG. 13. Combiners 221-224 combine the outputs from the various tile ports to form OLSA beams as shown. For example, each combiner 221-224 receives is physically connected to four tile ports. Additionally, a tile 235 can be connected to four separate combiners 221-224, as shown. Those skilled in the art will appreciate that this configuration is repeated over the entire antenna, which leads to complex physical interconnections among adjacent tiles.
FIG. 15 represents a conventional hardware configuration for digital beamforming in an OLSA antenna. Four manifolds 250 have four orthogonal ports each. However, in this case each orthogonal port is directly coupled to a receiver 260. The output from each port is converted to a digital signal. The ports from the four adjacent manifolds 250 can then be digitally combined in the digital beamforming unit 270 to form an OLSA beam. The digital beamforming requires 4(N+1)2 ports and 126N2R FLOPs per antenna, where R is the data rate from the digital receivers. Although the physical interconnections are reduced when compared to the architecture of FIG. 14, digital beamforming still requires costly antenna hardware configurations and digital processing for implementation according to conventional systems.
Additionally, on transmit, the OLSA architecture produces a natural trapezoidal taper across an aperture. This taper leads to significant power loss and taper loss, reducing the effective radiated power of the antenna. Partial channels along the circumference of the aperture reduce these losses at the expense of extra hardware (i.e., manifolds and receivers for analog beamforming, receivers and processing for digital beamforming).
To reduce manufacturing costs, antenna element support members that incorporate multiple elements (such as tiles) have been developed recently. Mass production of tiles results in significant cost savings when identical tiles are utilized. Minimizing the number of ports (and therefore receivers) per tile reduces the total cost of a tiled antenna, just as minimizing the element count for a given aperture size reduces the total antenna cost for a traditional planar array.
Accordingly, it is an object of the present invention to provide an improvement in tiled overlapping subarray antennas.
It is yet another object of the invention to provide an antenna that has a reduced number of ports and receivers per tile.
The foregoing and other objects are achieved by an array antenna comprising a plurality of tiles, each of which has two orthogonal ports and no physical interconnects to adjacent tiles. The array architecture also provides a plurality of receivers, each of which is individually coupled to orthogonal ports on said plurality of tiles. The array antenna includes a digital beamforming unit that receives outputs from said plurality of receivers and generates an overlapping subarray beam.
Additionally, the foregoing and other objects are achieved by a method for creating an antenna aperture with overlapped subarrays in two dimensions in an antenna comprised of a plurality of tiles. The method comprises: applying n phase offsets to a first port of a tile of the antenna thereby generating n beams, wherein n is an integer; adding said n beams to a second port of said tile thereby forming n partial overlapped subarrays; and combining n partial overlapped subarrays from n adjacent tiles to form one overlapped subarray.
Further scope of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that, while disclosing the preferred embodiment of the invention, the detailed description and specific embodiments (e.g., n=4, square antenna comprised of 64 square tiles) are provided by way of illustration only. Various changes and modifications coming within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description which follows.